The present invention relates to a novel 1,4-thiazine derivative represented by the general formula I and a pharmaceutically acceptable acid addition salt thereof, and processes for preparation thereof and a cardiotonic agent comprising said novel 1,4-thiazine derivative as an effective component: ##STR4## wherein R.sub.1, R.sub.2 and R.sub.3 respectively represent a hydrogen atom or a lower alkyl group.
Known processes for the synthesis of thiazine derivatives include a process starting with a thioglycol amide derivative as disclosed in Journal of the American Chemical Society, 70, 3517 (1948), a process starting with a tricyclic compound as disclosed in Japanese Patent Publication No. 16630/1967, and a process starting with a thiazolium compound as disclosed in Japanese Patent Publication No. 29182/1970. However, such technique cannot apply to the synthesis of 1,4-thiazine derivatives having a pyridinyl group at position 6.
Many pyridone derivatives and pyridazinone derivatives are known which possess pharmaceutical properties affecting the cardiovascular system. These are described in Japanese Patent Provisional Publication No. 48675/1977, Journal of Medicinal Chemistry, 17, 273 (1974) and Japanese Patent Provisional Publication No. 109771/1982 and so on. However, there is no teaching in the prior art of any cardiotonically active compounds having any chemical structure comparable to or suggestive of an instantly claimed compound.